Thrombotic thrombocytopenic purpura (TTP) is a life threatening disease characterized by diffuse platelet thrombi in the microcirculation, causing multiorgan dysfunction. Rare familial cases of TTP have constitutional deficiency of a VWF-cleaving protease whereas the majority of sporadic cases exhibit acquired autoantibodies to the protease. Our laboratory recently positionally cloned the gene responsible for familial TTP. This gene encodes ADAMTS13, a member of the ADAMTS family of zinc metalloproteases. The aim of this proposal is to map the immunodominant epitopes within human ADAMTS13 that are recognized by patient autoantibodies. To accomplish this goal a bacteriophage expression library of ADAMTS13 cDNA fragments will be constructed and screened with sera obtained from a large set of TTP patients. Major and minor sets of immunodominant epitopes will be defined and compared among different subsets of TTP patients. The observed epitope patterns will be correlated with disease severity, clinical outcome, and response to therapy. Characterization of these epitopes within ADAMTS13 will have direct applicability to the diagnosis and classification of TTP. A panel of recombinant fusion proteins containing ADAMTS13 epitopes will be developed during this proposal that should be directly applicable to rapid ELISA and Western blot assay. In addition, it may eventually be possible to develop therapeutic agents for the absorption of inhibiting antibodies as a primary therapy or adjunct to plasmapheresis.